Which of the following is a physical change?

Step 1: Analyze the question We need to find the option that involves a physical change, meaning no new substance is formed. Only the state or appearance changes, and the change is usually reversible.

Step 2: Recall the concept

• Physical change: no new substance forms (e.g., melting, freezing, dissolving sugar, boiling).
• Chemical change: a new substance forms (e.g., burning, rusting, reacting with water or air). Signs include heat or light release, gas production, a colour change, or a precipitate.

Step 3: Examine each option

• A. Exposing white phosphorus to air: White phosphorus reacts with oxygen and can ignite spontaneously, forming phosphorus oxides (P₄ + 5O₂ → P₄O₁₀). This is a chemical change.
• B. Burning kerosene: Combustion produces CO₂ and H₂O, which are new substances. This is a chemical change.
• C. Dissolving calcium in water: Calcium reacts with water: Ca + 2H₂O → Ca(OH)₂ + H₂. New substances form (calcium hydroxide and hydrogen gas). This is a chemical change, even though the word "dissolving" appears.
• D. Freezing ice cream: Liquid mix turns to a solid by cooling. The components are not chemically transformed. This is a physical change (a change of state).

A small caveat: ice cream is a complex mixture, but freezing it is essentially a change of state with no new chemical substances formed. It is clearly the best choice among the options.

Step 4: Answer The physical change is D. Freezing ice cream.

Exam tip: Watch for words like "dissolving" that sound physical. Always ask: "Is a new substance formed?" Reactive metals (Na, K, Ca) with water are always chemical changes, while dissolving sugar or salt is physical.